The present invention relates to sol-gel coatings which promote the adhesion of resinous material to a metallic surface. More particularly, the present invention relates to an anti-corrosive sol-gel coating which promotes the adhesion of resinous material to an aluminum or titanium surface.
Various coatings have been developed which promote the adhesion of one or more resinous layers to a titanium, aluminum, or other metallic surface such as that used as part of an airplane body.
Some of the coatings improve adhesion by utilizing a sol-gel film between the metal and the resin. The sol-gel films or sol coatings promote adhesion through a hybrid organometallic coupling agent at the metal surface. The metallic portion of the organometallic, which usually contains zirconium, bonds covalently with the metal. The organic portion of the organometallic bonds covalently with the applied layer of adhesive or matrix resin. In this manner, the organometallic based sol-gel creates a metal-to-resin gradient through a monolayer of the organometallic coupling agent.
The strength and durability of the sol coating depends upon chemical and micro-mechanical interactions at the surface of the metal involving, for example, the porosity and microstructure of the metal and the tendency of the sol coating to rehydrate. When properly implemented, the sol coatings provide high temperature surface stability for paint adhesion, adhesive bonding, or fabrication of structurally superior hybrid laminates.
One of the most effective of the organometallic sols, produced by The Boeing Company of Seattle, Wash., is a dilute solution of a stabilized alkoxyzirconium organometallic salt, such as Zr n-propoxide, and an organosilane coupling agent, such as 3-glycidoxypropyltrimethoxysilane (GTMS) that is used with an acetic acid catalyst. This particular formulation is known as Boe-Gel(copyright).
Although the Boe-Gel(copyright) sol has superior adhesive properties, the Boe-Gel(copyright) sol formulation has only minimal anti-corrosion characteristics. In most situations in which the Boe-Gel(copyright) sol is used to adhere a layer of paint, adhesive, or other compound to the metallic surface of an airplane, the material which is adhered to the metallic surface has anti-corrosive properties, and thus protects the metallic surface from corrosion. Problems arise, however, in situations in which the sol-gel coating may become exposed to the environment or may be used alone, in the absence of an anti-corrosive layer.
In the past, chromate compounds have been used to impart anti-corrosive properties to coatings such as the sol-gels discussed above, but chromates have recently come under scrutiny by the Environmental Protection Agency (EPA) for possible health concerns, and new governmental regulations seek to phase out the use of chromates as corrosion inhibitors.
What is needed is a chromate free sol-gel composition providing the same superior metal to resin adhesive and barrier properties as the alkoxyzirconium organometallic salt gels, but which provides the metallic surfaces with significant anti-corrosion properties without the need of an additional anti-corrosive layer.
The invention is a chromate-free coating which provides superior adhesion, barrier protection, and anti-corrosion properties to a metal surface, and more particularly to an aluminum or titanium surface. The invented coating is an anti-corrosive sol-gel which produces an adhesive film interface between the metal surface and an organic matrix resin or adhesive, and has anti-corrosive properties providing protection for the underlying metal substrate. The invented sol-gel forms a film which promotes adhesion through a hybrid organometallic coupling agent at the metal surface. The anti-corrosive properties of the sol-gel prevent corrosion damage to the covered metal surface even when the sol-gel coating is not covered by an additional resin layer.
The sol-gel is a combination of an organometallic salt, an organosilane, and one or more compounds having borate, zinc, or phosphate functionality. The organometallic component may be selected from a large group of equivalent compounds but is preferably an alkoxyzirconium salt, and most preferably Zr (IV) n-propoxide. The organosilane may also be selected from a large group of equivalent compounds, but is preferably 3-glycidoxypropyltrimethoxysilane (GTMS). Compounds with any borate, zinc, or phosphate functional groups may be used, but phosphates are preferred.
The functional components of the sol-gel are combined with an organic acid prior to application to metal. The organic acid, preferably acetic acid, acts as a catalyst and as a reaction rate regulating agent.
The invention provides a chromate free sol-gel composition providing the same superior metal to resin adhesive and barrier properties as previous alkoxyzirconium sol-gels, but which provides metallic surfaces with significant anti-corrosion properties without the need of an additional anti-corrosive layer.